The invention relates to an arc extinguishing chamber whose side walls are made of composite material, and to a switchgear device comprising such a chamber.
Low-voltage circuit breakers of high ratings more often than not comprise separable contacts arranged at the entry of an arc extinguishing chamber. When separation of the contacts takes place caused by a trip device following an overcurrent, an electrical arc arises between the contacts and is propagated in an arc extinguishing chamber designed to absorb the energy of the arc while maintaining its voltage. The chamber comprises a plurality of separators arranged transversely to the arc and designed to break the arc down into fractions. This fractioning enables the voltage of the arc to be increased and the arc to be cooled by heat exchange with the separators. The separators are supported by two side walls of the chamber, arranged facing one another perpendicularly to the separators. These side walls essentially have to perform mechanical securing of the separators and electrical insulation.
The chamber is subjected to very high thermal, mechanical and electrical stresses: to give a good idea thereof, a current of 200,000 amperes maintained for 4 ms at an arcing voltage of 500 Volts gives off an energy of 400 kJ. The plasma column forming the arc can reach a temperature situated between 4,000 and 20,000 Kelvins. The separators are subjected to electrodynamic forces during breaking tending to separate them from one another. The pressure in the arc extinguishing chamber can at the same time reach 1.4 MPa.
The side walls have to withstand these stresses without becoming conducting themselves and without giving off a low dielectric strength gas.
Traditionally, the walls are formed by a stratified material composed of successive layers of thermosetting resin reinforced by fiber glass. The glass fibers give the walls their mechanical strength. However glass fibers contain low ionization potential elements. Experience shows that when these glass fibers are subjected to high temperatures, the elements having a low ionization potential inside the fibers ionize and hamper the arc extinguishing process, in particular for voltages in excess of 400 Volts. In addition, molten glass beads appear at the surface due to ablation of the resin and foster adhesion of metallic particles given off in the chamber by melting of the separators. The surface resistance of the walls, taken between two points both of which are close to one of the separated contacts, therefore decreases during breaking. For these reasons, the risk of breaking failure is high.
To overcome this problem, the document FR-2,616,009 proposes a three-layer composite stratified structure. The external layers are formed by a multitude of linen fibers impregnated with melamine resin whereas the internal layer is constituted by a multitude of woven glass fibers impregnated with melamine resin. The layer comprising the glass fibers gives the structure its rigidity whereas the superficial layer comprising linen fibers remains non-conducting even during and after exposure to the arc. This stratified material proves satisfactory in applications where it is only exposed to the arc on the side where its layer comprising linen fibers is situated. The material does on the other hand present some problems in an architecture requiring that an edge of the side wall be exposed to the arc. Such an architecture is for example encountered in the case of a circuit breaker comprising, for a given phase, two poles connected in parallel, each pole being provided with an arc extinguishing chamber, the arc extinguishing chambers being connected to one another by a communication orifice made in the adjacent side walls of the two chambers and enabling circulation of the gases. A circuit breaker of this type is described in the French Patent Application bearing the national registration number 98/06206. With such a cutting of the stratified material plate, the layer comprising glass fibers is flush with the surface of the edge, resulting in a certain vulnerablity in this zone. It is naturally possible to deposit an additional layer comprising linen fiber to specifically protect this zone, but this solution is costly.
It has moreover been proposed in the document DE-A-43 22 351 to replace the melamine-based thermosetting resins reinforced with cotton or linen cellulose fibers by a polyamide thermoplastic polymer matrix containing a cellulose material coated with a hardened melamine-formaldehyde resin, in which the polyamide and coated cellulose material are present in a ratio of 6:1 to 1:1. The material used is supposed to give dielectric properties at least equal to those of thermosetting materials, and better mechanical properties. However, experience shows that the thermoplastic nature of the material gives rise to problems from the temperature withstand point of view, in particular when progressive diffusion of the heat stored by the metallic separators takes place, during and after breaking, i.e. in practice about 30% of the breaking energy. As the polyamide of the walls tends to soften when the temperature rises, it undergoes deformations rapidly making the chamber unusable. This is why this solution is not applicable to circuit breakers with high ratings.